What is meant by free pawn?

a pawn that has no opponent's pawn in front of it, so it cannot be stopped by another pawn 

So free pawns cannot be captured or can be captured with certain risks and consequences?

I think you are confusing many different terms. What you wrote here is a "poisoned pawn". What you wrote in the opening post is a "passed pawn". Those are completely different.

What I mean is how to prevent a pawn from being free before it reaches the promotion square? Because sometimes I have difficulty preventing the opponent's pawn from being promoted.

A pawn that has no pawn of the opposite color on its file or on an adjacent file is a "passed pawn". If there is an opposite colored pawn on an adjacent file then it is a "candidate passed pawn".

Can you provide the context in which the term "free pawn" was used?